The present invention pertains to devices for measuring the altitude of an aircraft, and in particular to an altimeter having a digitizer correctable to barometric pressure.
Air traffic control systems utilize transponders installed within an aircraft to monitor the position and altitude of aircraft flying within controlled airspace. The transponder is a transceiver which, when interrogated by an air traffic control radar station, replies with an identification code input by the pilot and the pressure altitude of the aircraft referenced to sea level (29.92 inches of mercury). This pressure altitude information is provided by an altitude measuring and reporting device. Three fundamental types of altitude reporting devices are in use today: (1) air data computers, (2) encoding altimeters, and (3) altitude digitizers. The air data computer is a device which uses various sensors and micro processing techniques to determine altitude, airspeed, position and numerous other parameters which affect aircraft performance. Air data computers are highly accurate, extremely sensitive, and very expensive to purchase and maintain.
Encoding altimeters are much less expensive. FIGS. 1-3 illustrated cutaway side elevation, end elevation, and opposite end elevation views, respectively, of a conventional prior art encoding altimeter, generally designated 500. Altimeter 500 is an electromechanical device which uses a pressure sensitive mechanical movement in the form of an aneroid 502 to sense outside air pressure. Aneroid 502 is in a sealed housing 515 connected to a static pressure line through a port 512. The aneroid 502 drives a series of gears which in turn drive a pointer 504 and a numbered counter drum 503 which provides the pilot with an altitude reading. Pointer 504 makes one revolution for each 1000 feet of altitude. Counter drum 503 indicates the altitude of the aircraft to the nearest 100 feet. Pointer 504 and counter drum 503 are both used to read altitude. Counter drum 503 provides the most significant digits and pointer 504 provides the least significant digits. The aneroid 502 is also mechanically linked 506 to a shaft angle encoder 508 which provides a digitized representation of the aircraft""s altitude to a transponder via a connector 510.
Altimeter 500 has two counter drums 505 and 509 which the pilot sets to the barometric pressure provided by air traffic control over the radio. Counter drum 505 indicates sea level barometric pressure in isobars and counter drum 509 indicates sea level barometric pressure in inches of mercury with 29.92 inches being nominal. When the pilot hears the barometric pressure over the radio, he looks to see if the readings on the drums 505 and 509 are correct. If not, he uses a barometric pressure reference correction control in the form of knob 507 to set the readings to the barometric pressure. When knob 507 is turned, gears inside mechanically move the readings on the drums 505 and 509. In FIG. 1, the barometric pressure is represented as being 29.92 inches of mercury and the altitude is 5460 with 5400 feet shown on drum 503 and 460 feet shown by needle 504.
FIG. 4 illustrates the changes which result when altimeter 500 is set to a different barometric pressure. In FIG. 4, the pilot has moved knob 507 to increase the setting of barometric pressure on drum 509 to 30.00 inches of mercury. Indicated altitude is changed thereby to 5532 feet with 5500 feet shown on drum 503 and 532 shown by needle 504. Counter drum 505 showing isobars has also moved to 1016. Thus a change of 0.08 inches of mercury in barometric pressure results in a 72 foot correction in indicated altitude.
It should be noted that this altitude correction is not made in the shaft angle encoder 508. This is because air traffic control prefers to receive a raw altitude indication from aircraft instead of one which is subject to errors created by the pilot. Air traffic control adjusts the raw altitude indication provided by the transponder from the shaft angle encoder using the barometric pressure at the location of the aircraft to establish the true altitude of the aircraft.
FIGS. 5 and 6 show another form of prior art encoding altimeter generally designated 600, which is a Series 5035 encoding altimeter sold by United Instruments, Incorporated, 3625 Comatara Avenue, Wichita, Kans. 67226. FIG. 5 is a front, right side, exploded perspective view and FIG. 6 is a front elevation view. This encoding altimeter has a frame 602 which carries an altimeter section 604 and encoder 606 and rotates inside a housing 608. The previous encoding altimeter of FIGS. 1-4 has multiple gears and shafts for adjusting the setting of the barometric pressure. The present encoding altimeter has only two gears: a ring gear 610 at the perimeter of the frame 602 and a driver gear 612 connected to knob 614. When the pilot turns the knob, the ring gear turns the frame in relation to a face 616 which is fixed to the housing 608. A pressure setting dial 618 fixed to the front of the frame shows through a window 620 in the face to show the pilot the barometric pressure setting.
The encoder 606 is not adjusted by the pilot when he moves the knob 614. As in the previous encoding altimeter, the encoder 606 only provides a raw pressure indication to the transponder which is interpreted by air traffic control using the barometric pressure at the location of the aircraft.
Altitude digitizers may also be completely independent of the altimeter and may either be electromechanical or solid state in nature. The FAA requires that such an altitude digitizer be calibrated to within xc2x1125 feet of the primary altimeter viewed by the pilot. U.S. Pat. No. Re. 29,436 illustrates an electromechanical digitizer mechanically linked to an aneroid. A shaft angle encoder converts angular position into a digital code representing altitude. Here again, this device is expensive and difficult to calibrate. Solid state digitizers are disclosed in U.S. Pat. No. 4,106,343, and Model SSD 120 Altitude Encoder/Digitizer, sold by Trans-Cal Industries, Inc., 16141 Cohasset Street, Van Nuys, Calif. 91406. These devices have a solid state pressure transducer which converts air pressure to voltage which is then converted to a digital code representing altitude. These devices are much less expensive than electromechanical shaft angle encoding digitizers.
FIG. 7 is a block diagram of a solid state digitizer in a typical aircraft system. The digitizer and altimeter are connected to different portions of the static pressure line. The output of the solid state digitizer is routed in parallel form to the aircraft""s transponder, and in serial form to a Global Positioning System (GPS) navigational computer, where it serves as a backup altitude signal should the GPS solution become degraded when less than four satellites are available for determination of an accurate altitude
The present invention is directed to an improved altitude measuring device including an altimeter having a digitizer output corrected for barometric pressure. The present invention has the following advantages over previous devices:
since the digitizer output is corrected for barometric pressure, the output can be used by each of the receiving navigation system devices without the pilot having to adjust the barometric pressure on each device; and,
the digitizer altitude output is the same as the indicated altimeter altitude.
In accordance with a preferred embodiment of the invention, the altimeter having a correctable digitizer includes a conventional altimeter of the aneroid type, having a housing, a pressure sensitive mechanical movement disposed within the housing, and a barometric pressure correction control, such as a knob, disposed outside the housing. A digitizer is also located within the altimeter housing and generates an aircraft altitude output value based upon sensed air pressure. An input device is connected between the barometric pressure correction control and the digitizer, so that when the barometric pressure correction control is changed, the input device generates a barometric pressure correction signal which is delivered to the digitizer. The barometric pressure correction signal is used to modify the aircraft altitude output value of the digitizer to result in a corrected aircraft altitude output value which is routed to one or more external navigation systems.
In accordance with an important aspect of the invention, the barometric pressure correction control includes a knob and the input device includes a rotary shaft input which is mechanically linked to the knob so that when the knob is rotated, the shaft input rotates.
In accordance with an important feature of the invention, the input device includes a rotary potentiometer and an analog-to-digital converter.
In accordance with another important aspect of the invention, the corrected aircraft altitude output value of the digitizer is a serial data stream.
In accordance with another important feature of the invention, the corrected aircraft altitude output value makes it unnecessary for the pilot or crew member to adjust each of the external navigation systems for actual reference barometric pressure.
In accordance with another important aspect of the invention, the barometric pressure correction signal ensures that the altitude delivered to the external navigation system by the digitizer is the same as the altitude seen by the pilot on the altimeter.